Suter. — Tin Origin, of a ~Sew Species by Isolation. 281 



example, the P. hongii ambagiosus gradually losing its incrassate notched 

 peristome, the shell getting somewhat thinner, forming the species known 

 as P. hongii ; and the farther these snails have gone from the north south- 

 wards the more simple the outer lip has grown. However, we must not 

 forget that the species of Placostylus are polymorphic, and variations in the 

 characters of the shell are not rare. Why heavy, massive snails should go 

 on slowly evolving lighter and more simply constructed shells I cannot 

 explain ; the fact only remains. . 



I beg leave to bring forward here a hypothesis regarding a similar de- 

 velopment in marine shells. The genus Pngnellus Conrad has nearly the 

 whole of its shell covered by a thick calcareous deposit, so that only a small 

 part of it can be seen. Species of this genus are found in the Cretaceous 

 of Austria, southern India, North America, North Africa, Borneo, Chile, 

 Patagonia, and in the older Tertiary of New Zealand (Wangaloa). We have 

 in New Zealand the genus Conchothyra Hutton, which is nearly allied to 

 Pngnellus, but has a tremendous calcareous covering, hiding nearly the 

 wtiole shell. This I consider as an extreme form of Pngnellus, and it is 

 peculiar to the Cretaceous and older Tertiary of New Zealand. Pngnellus 

 is followed by Tylospira, which has part of the body-whorl and spire covered 

 with a thick enamel. There is one Recent species in Australia ; it also 

 occurs in the Miocene of Australia and New Zealand. At last we come 

 to the most simple genus of the series, Struthiolaria, which has only a thin 

 broad inner lip, and a thickened sinuous outer lip. It is found in the Cre- 

 taceous, Eocene, and Oligocene of Patagonia ; Miocene of Australia ; Miocene, 

 Pliocene, and Recent in New Zealand ; &c. I am well aware that Cossmann 

 includes Pugnellus in the family Strombidae, and Tylospira with Struthio- 

 laria in the family Struthiolariidae. Zittel, however, unites them all in 

 the former family. My opinion is that we have to class all these genera 

 either under Strombidae or under Struthiolariidae, and I am more'in favour 

 of the latter, as I consider Pugnellus to be the ancestor of Tylospira and 

 Struthiolaria. 



To return to our subject. Lesson and Martinet, in their work Les 

 Polynesiens, say that Bulimus hongii was plentiful amongst flax-bushes near 

 the North Cape, and that Bulimus vibratus was abundant on the Three Kings. 

 The latter species (Placostylus fibratus Martyn) is a somewhat related New 

 Caledonian species, and our P. hongii was sometimes, though wrongly, going 

 under that name. However, the use of two specific names shows that at 

 one time it was known that the Three Kings harboured a species distinct 

 from that on the mainland, but the curious thing is that not a single col- 

 lection in New Zealand contained an example of the Great King Placostylus. 

 If I had not made a note of Lesson and Martinet's statement, had not asked 

 Captain Bollons to look for Placostylus on the Great King, and if he had not 

 been successful in finding it, this interesting shell would still be unknown. 



The characteristic features of Placostylus bollonsi are its rather thin 

 large shell, its simple aperture, its network-like sculpture, the large obtuse 

 broadly rounded apex or protoconch with its peculiar ornamentation, the 

 total absence of a spermatheca in the animal, and the large calcareous egg, 

 which involves a large embryonic shell. The thimble 7 shaped large proto- 

 conch with its fine oblique axial riblets distinguish P. bollonsi from all the 

 other species of the genus. 



When Placostylus reached New Zealand from the north-west the Three 

 Kings were united with what is now the North Island, and must have been 

 populated by the same species we still find in the north, P. hongii am- 



